This invention relates to apparatus and method for forming a gloss finish on or transparentizing resin-coated webs or sheets. Such material may be developer or receiver sheets as used in conjunction with photosensitive imaging systems employing microcapsules to provide visible image upon contact with a color precursor. Such imaging systems are the subject of commonly assigned U.S. Pat. Nos. 4,399,209; 4,416,966; and 4,440,846. The substrate may, for example, be paper or overhead projection base material (OHP).
The imaging systems described in the aboveidentified patents are characterized by an imaging sheet including a layer of microcapsules which contain photosensitive composition in the internal phase which is image-wise exposed to actinic radiation. U.S. Pat. No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer or receiver sheet prior to being subjected to a rupturing force. Upon passing through a pressure developer, the microcapsules rupture and image-wise release the internal phase whereupon the color precursor migrates to the developer sheet where it reacts with the dry developer and forms a color image.
Developer or receiver sheets or webs are provided with a resin on the surface which may be glossed or transparentized by the application of heat and pressure, while supporting the sheet or web with the resin against a smooth or highly polished surface. The coalescing resin coating is found to improve the perceived image quality by increasing color saturation. The heated surface may be that of a heated convex plate, a roll, or a belt.
The resin coating may be a vinyl copolymer as disclosed in commonly assigned copending U.S. patent application Ser. No. 086,059 filed Aug. 14, 1987, now U.S. Pat. No. 877,767, or a phenolic resin as disclosed in commonly assigned U.S. patent application Ser. No. 073,036 filed June 14, 1987, now U.S. Pat. No. 4,859,561.
Such resins as currently used are obtained in a dispersed form and are roll and/or blade coated. Commonly the resin is finely ground or finely divided and dispersed in a carrier oil, and the resulting coating on the substrate, has a rather opaque, white or milky appearance. In the processing or glossing stage, it is necessary to transparentize this coating and provide thereon a suitable smooth high gloss surface condition.
The milky or opaque appearance of the coating, prior to transparentizing, is believed to be due to the reflection and refraction of light at the uneven surface, and additionally due to the breaking up of the light at the interfaces between the carrier and the dispersed particles. The latter may be viewed as small globules or spheres of particles under a microscope at between 200 and 500 power magnification.
The resin has a glass transition temperature T.sub.g in range in which the coating constituents, each having approximately the same index of refraction, coalesce into a common surface where the individual parts can no longer be visually or optically distinguished, and one which has a minimum of residual haze and a maximum of gloss, as may be measured on a conventional gloss meter.
A belt-type glosser for applying a gloss finish to such developer sheets is described in commonly assigned U.S. Pat. No. 4,761,311, the disclosure of which is incorporated herein by reference. In the above-identified patent, a polymer belt forms the glossing surface for the softening and finishing of the resin coating by the transfer of heat and pressure from a heated surface in running contact with the opposite side of the belt.
Typical thermoplastic developer resins which are capable of forming a film upon the application of heat and pressure are rather tacky compared to dry silver film, conventional film and xerographic material. Thus, the release conditions of the developer sheet surface from the belt can be critical.
Present glossing or transparentizing systems generally apply a combination of heat and pressure to the resin coating. The resin may be heated by way of conduction directly by contact under pressure with a heated glossing or casting surface. This means of glossing is more efficient than heating through the back side of the sheet or web material, but has the drawback that at relatively high temperatures, the resin tends to stick to most heated surfaces.
Another prior process of glossing employs the radiation heating of the resin surface. This process is relatively inefficient, requires high temperatures, and can cause resin damage at high energy densities.